Converters are used in electrical systems for transforming an input voltage, which can be a first DC voltage or a first AC voltage of a first frequency, into an output voltage, which can be a second DC voltage or a second AC voltage of a second frequency.
For example, if the converter is used as an inverter, the input voltage can be a DC link voltage and the output voltage can be used for supplying an electrical load such as an electrical machine with electrical current. Conversely, a converter can be used as a rectifier in which case it is connected to an electrical grid and transforms an input AC voltage into a DC output voltage.
Nearly all converters include semiconductor switches which generate the output voltage, for example for each phase of the electrical system. The switches are controlled by a controller that determines the next switching transition and applies this switching transition to the switches. The switching transition can include a set of switching states (i.e. whether the respective switch is opened or closed) and the time instant (i.e. the time point) at which these switching states are applied to the converter.
There are several possibilities for how these switching transitions can be generated by the controller.
For example, model predictive direct torque control (MPDTC) can be used for controlling internal states of the electrical system while generating the switching transitions. Internal states of an electrical drive can be the torque and the electromagnetic fluxes in the motor. In MPDTC, switching sequences, i.e. sequences of switching transitions over a certain switching horizon are optimized in real time. For example, the corresponding torque, stator flux and neutral point trajectories can be computed using an internal machine model and then an optimal switching sequence can be chosen that features the lowest switching losses or the lowest switching frequency. The first switching transition of the switching sequence can then be applied to the converter and the next sequence can be optimized online.
Direct torque control can achieve a very fast torque response, but it can lead to relatively high values of harmonic distortion of the stator currents and of the electromagnetic torque of the controlled machine for a given value of the switching frequency or of the switching losses.
Another possibility is the use of optimized pulse patterns (OPPs). Generally, an optimized pulse pattern can be a sequence of switching transitions that has been optimized with respect to a certain optimization goal. For example, optimized pulse patterns can be computed offline for all modulation indices and switching frequencies of the motor, or of any other physical system that acts as the load of the converter, and can be optimized such that the overall current distortion for a given switching frequency is minimal. The controller can select an optimized pulse pattern from a look-up table in which the optimized pulse patterns are stored and can apply the switching transitions of the selected optimized pulse pattern to the converter. However, the application of optimized pulse patterns can lead to very long transients and to harmonic excursions of the stator currents when changing the operating point: even very small changes of the operating point can lead to comparatively high excursions of the harmonic current. Such harmonic excursions can be quantified as current errors or, equivalently, as flux errors.